With the advent of acrylate based bone cements, joint replacements and other surgical procedures using bone cement have become commonplace.
In joint replacement procedures, a prosthetic implant is fixed to the patient's bone. Such procedures begin with the surgical removal of the diseased portion of a bone. The most often used technique for setting a prosthetic implant proceeds with the preparation of the bone tissue by making a cavity therein. The cavity is shaped to receive a securement portion of a prosthetic implant. Also, a number of shallow holes may preferably be made in the surface of the bone tissue adjacent to the site where the prosthetic device will ultimately be fixed. These shallow holes project out from the cavity and provide a void into which bone cement may subsequently flow and cure. The surfaces of the bone are then thoroughly cleansed of all blood, fatty marrow tissue, bone fragments, and the like. The bone cement is then placed in the attachment site by any known method. The bone cement ultimately fills the interstices between the bone and the securement portion of the prosthetic implant, thereby facilitating a strong mechanical interlock between the bone, the bone cement and the prosthetic implant. To facilitate a strong mechanical interlock, it is desirable that the bone cement is able to freely flow into the porous recesses of the bone and any surgically produced holes projecting from the cavity.
Conventional acrylate based bone cements are widely used by surgeons. These conventional acrylate based bone cements are generally supplied to the surgeon as two separate components, a liquid component and a powder component. The liquid component of the bone cement generally comprises a liquid mixture with monomeric methyl methacrylate as the principal constituent. The powder component of these bone cements generally comprises a dry powder mixture with the primary constituent being a [methyl methacrylate-styrene] copolymer.
The recommended manner by which the liquid and dry components of conventional bone cements are mixed involves emptying the powdered component into a sterile container followed by addition of the liquid component. The components are then mixed thoroughly until polymerization commences. The specific mixing time depends on the bone cement used; the atmospheric conditions in the operating room, i.e., the temperature; and the method to be used to administer the bone cement. For pressurized administration, the components are mixed for a period of time before being loaded into a suitable sterile syringe while still relatively non-viscous for injection into the prepared area. Alternatively, for manual administration, the components should be mixed until the mixture develops a dough-like consistency which does not stick to a surgical glove. The bone cement may then be formed into a suitable shape for placement in the prepared area.
If administered before the degree of polymerization of the bone cement has proceeded to a suitable extent, the bone cement will be too fluid, difficult to handle and may cause overflow problems wherein the bone cement enters undesirable locations inside the patient where it must latter be removed to avoid complications. If administered after the degree of polymerization is too advanced, the bone cement will be too viscous and will not flow into all the interstitial areas and porous recesses of the bone to which the prosthetic implant is to be fixed. Furthermore, the bone cement may cure before the surgeon has sufficient time to properly align the prosthetic implant. Thus, the mixing time is an important variable in prosthetic implant procedures utilizing bone cements.
The mechanical interlock between the bone, the bone cement and the prosthetic implant is prone to deteriorate with time. Namely, over time, prosthetic implants may show signs of loosening as a result of a break down in the mechanical interlock. Loosening most often occurs at the interface between the bone and the cured bone cement, i.e., at the bone/cement interface. Bone cement failure is believed to be the primary cause of loosening. Specifically, conventional bone cements exhibit a tendency to fail by brittle fracture and fatigue, thereby losing its ability to transmit load from the prosthetic implant to the bone. This increased stress ultimately results in the loosening of the prosthetic implant with concomitant joint dysfunction and patient pain.
Historically, upwards of 20% of all hip joint prosthetic implants require maintenance after about 10 years of service. Maintenance ordinarily involves the surgical removal of the prosthetic implant and the cured bone cement. Removal of the bone cement is difficult and time consuming, requiring the surgeon to grind, pick and scrape the bone cement from the interstitial areas and porous recesses in the bone. Furthermore, the surgical removal of the implant like any other surgical procedure is visited with threat of infection and/or other complications resulting from surgery. Consequently, this high incidence of required maintenance makes desirable the development of improved designs for the prosthetic implants and the composition of the bone cements used to fix them to a bone. Accordingly, improved bone cement compositions which increase the useful life of an implant are desirable.
It is believed that the useful lifetime of the affixation of a prosthetic implant is a function of the fatigue strength of the bone cement used to affix the implant. Accordingly, bone cements having improved fatigue strength are desirable.
Furthermore, regardless of the method of administration used, the administering surgeon determines how long to mix the constituents before administration based largely upon the surgeon's knowledge and experience with the given bone cement and its historic handling characteristics. Surgeons who frequently perform prosthetic implant procedures have generally become familiar with the specific handling characteristics of the given bone cement they use. Particularly, most experienced surgeons are able to recognize when the bone cement is ready for administration based upon its consistency. This ability to recognize when bone cement is ready for administration is a skill that surgeons develop over time with continued use and experience with a given bone cement. Accordingly, it is desirable to develop improved bone cement compositions which exhibit handling characteristics which are identical or nearly identical to the bone cements with which surgeons are familiar.
Previous attempts to improve the fatigue strength of conventional bone cements through the addition of materials such as carbon fibers, glass fibers, silica, alumina, boron fibers, and the like have proven to be largely unsuccessful for a variety of reasons. For example, such fibers have been observed to cause a drastic reduction in the flow characteristics of the bone cements into which they are incorporated. Specifically, the fibers have been observed to block the flow of the bone cement into the interstitial areas and porous recesses in the bone and any surgically produced holes projecting from the cavity into which the prosthetic implant is to be fixed. This reduction in flow characteristics is believed to result in a poor mechanical interlock between the bone and the prosthetic implant.